The pre-treatment of landfill leachate prior to its co-treatment in the municipal plants of waste water processing could represent an appropriate and cost-effective solution for its management. Pre-treatment is necessary especially to remove heavy metals, which may be transferred to the excess sludge preventing its valorisation. In the present paper, we propose a chemical-physical pre-treatment of leachate using four different granular reactive media able to selectively remove the contaminants present in the leachate. The efficiency of these materials was investigated using synthetic leachate through batch tests and a column test. In the latter case the four materials were placed in two columns connected in series and fed an under constant upward flow (0.5 mL/min). The first column was filled half (50 cm) with a granular mixture of zero valent iron (ZVI) and pumice and half (50 cm) with a granular mixture of ZVI and granular activated carbon (GAC). The second column, which was fed with the effluent of the first column, was filled half with zeolite (chabazite) and half with GAC. Heavy metals were mainly removed by the ZVI/pumice and ZVI/GAC steps with a removal efficiency that was higher than 98, 94 and 90% for copper, nickel and zinc, respectively, after 70 days of operation. Ammonium was removed by zeolite with a removal efficiency of 99% up to 23 days. The average reduction of the chemical oxygen demand (COD) was of 40% for 85 days, whereas chloride and sulphate removal was negligible.

Introduction

One of the most important issues for the overall sustainability (economic and environmental) of a modern landfill is leachate management; in fact, leachate is a complex and highly polluted matrix containing a large amount of dissolved organic matter, which is biodegradable or refractory to biodegradation (e.g. humic acid), and of inorganic compounds such as: (i) light metals (Al, K, Na, Mg, etc.); (ii) heavy metals and metalloids (As, Ca, Cd, Cu, Fe, Pb; Zn, etc.); (iii) anions (Cl−, NO2 −, NO3 −, SO4 2−, PO4 3−, S2− etc.), and (iv) NH3 (Fan et al., 2006; Kjeldsen et al., 2002; Qasim, 2017; Slack et al., 2005; Wiszniowski et al., 2006). Whereas anions and light metals are generally present in non-toxic concentrations, the toxicity of heavy metals and As may be considered a threat (Heyer and Stegmann, 2002; Wiszniowski et al., 2006). According to landfill age, leachate is generally classified as young or stabilised (or mature):young leachate generally presents low pH values (b6.5) and higher values of organic matter content and biodegradability (i.e. COD up to 50.000 mg/L and ratio between biological and chemical oxygen demand – BOD/COD N 0.4) and of heavy metals. Old or stabilised leachate usually presents higher values of pH (N7.5) and NH4–N (N400 mg/L) and lower values of COD (b3000–۴۰۰۰ mg/L), of the BOD/COD ratio (down to 0.1) and of heavy metals (adapted from Gandhimathi et al., 2013, Foo and Hameed, 2009, Renou et al., 2008). Conventional landfill leachate treatments can be classified into four major groups: i) recycling of leachate into the landfill body, ii) combined treatment with domestic sewage in external wastewater treatment plants (WWTPs), iii) biodegradation: aerobic and anaerobic processes, and iv) chemical and physical methods: chemical oxidation, adsorption, chemical precipitation, coagulation/flocculation, sedimentation/flotation and air stripping (Hermosilla et al., 2009; Kurniawan et al., 2006; Renou et al., 2008).